Synthesis of formaldehyde from CO₂ catalyzed by the coupled photo-enzyme system

Using the reduced nicotinamide adenine dinucleotide (NADH) as coenzyme, formate dehydrogenase (FDH) and formaldehyde dehydrogenase (FADH) can catalyze the reduction of CO₂ to formaldehyde. In this study, a hollow fiber membrane module was used as the enzyme-bearing reactor and gas distributor, and then coupled with a UV/TiO₂ photocatalytic NADH regeneration system. First, photocatalytic CO₂ reduction with varied pH value and electron donor was studied as a control. Then, CO₂ reduction catalyzed by the photo-enzyme coupled system was investigated and the limiting step of the reactions was analyzed. At last, the ratio of FDH and FADH, and cofactor concentration were optimized, and the operation stability was examined as well. The results show that the optimal pH value for the production of formaldehyde by photocatalysis and photo-enzyme coupling catalysis is respectively 6.5 and 7.0 with H₂O as electron donor. At pH 7.0, the initial reaction rate is 0.035 and 0.104 mmol/(L·h), and the formaldehyde production after 5 h is 0.117 and 0.393 mmol/L, proving the enzymatic reduction of CO₂ is predominant in the coupled system. With EDTA as electron donor, the optimal pH value is 5.5 for the coupled system, and the formaldehyde production is much higher than that with H₂O at all pH values, however, part of them might come from EDTA. For the sequence reactions, the second step is a rate-limiting step, and cofactor regeneration is vital for enzyme catalysis. The optimal loading ratio of FDH and FADH is 1:0.3, and with the addition of 1 mmol/L NAD⁺ or NADH, the yield of formaldehyde can reach 3.83% and 6.47% after 4 h. The photo-enzyme coupled system shows a good manipulation stability during 48 h.